The end of an era - ERS-2 retires after 16 years

It was a moving event. On July 4, 2011 at 09:17 Central European Summer Time (CEST, UTC+02:00) the moment had arrived: the DLR ground station in the Antarctic received for the last time data from the European earth observation satellite ERS-2. Shortly before, at 06:28 CEST, the European Remote Sensing satellite sent the last data to Chetumal in Mexico, where DLR also operates a ground station.

Since the afternoon of July 4, 2011 the satellite’s familiar signs of life were absent. After 16 years and 84,720 orbits the earth observation instruments on board ERS-2 were turned off. This satellite considerably outlived its design life. After several mission extensions, colleagues at the European Space Agency ESA finally retired the satellite. Two days later orbit maneuvers were initiated to reduce the ERS-2 orbit altitude from the customary 800 km down to approximately 570 km. This is being done by ESA so that ERS-2 will burn up in the atmosphere within the next 25 years, in accordance with the European Code of Conduct on Space Debris Mitigation.

The 20-year history of the DLR ground station on the northern tip of the Antarctic Peninsula is closely associated with the earth observation satellites ERS-1 and ERS-2. The German Remote Sensing Data Center (DFD) set up its Antarctic station in 1991 specifically to receive data from these two satellites. This was accomplished together with the then Institute of Applied Geodesy, today the German Federal Agency for Cartography and Geodesy, and with the help of partners in Chile. The German Antarctic Receiving Station (GARS) O'Higgins recorded the first data from ERS-1 shortly after its launch in September 1991.

Much of our current knowledge about the changes in the Antarctic ice sheet comes from ERS-1 and ERS-2. Particularly data from the on board Synthetic Aperture Radar (SAR) provided completely new insights thanks to this instrument's ability to record independently of weather and light conditions. But since the satellites did not have the capacity to store the collected radar data on board, the data had to be received right where it was collected, in the inhospitable frozen wilderness of Antarctica. In the end, after an ERS-2 tape recorder ceased functioning, also data from the other ERS instruments could only be received via the GARS O'Higgins ground station.

So our knowledge about the disintegration of the shelf ice regions around the Antarctic Peninsula, the reduction in ice thickness in the western Antarctic, the borderline between inland ice and floating shelf ice (the so-called grounding line), and the Antarctic ozone hole is also based on data collected during operation of the O'Higgins receiving station. Up until the earth observation sensors were turned off on July 4, 2011, DFD colleagues in the Antarctic were recording almost ten ERS-2 passes daily.

By the end of February 2011 ESA had modified the satellite orbit so that SAR images of the Antarctic could be recorded with the exact same viewing angle every three days, instead of every 35 days as previously. This was a unique opportunity for differential SAR interferometry, which involves comparing the radar signal run-time differences for three acquisitions, making it possible to generate terrain models and derive ice movement. Without the year-round availability of the GARS O’Higgins station this data "gold mine" from the last mission phase of ERS-2 would not have been possible to exploit.

The last historic ERS-2 data from GARS O'Higgins were flown out from the Antarctic on July 6, 2011. Using a snowmobile, a team from the neighboring Chilean military base "General Bernardo O'Higgins" brought the data tapes to the glacier runway located about three kilometers away. These data were then transported to King George Island on a Chilean air force Twin Otter DHC-6 aircraft, and from there a Hercules C-130 plane of the Brazilian air force transported them to Punta Arenas. As commercial air freight they will arrive in Oberpfaffenhofen for processing and archiving at the German Remote Sensing Data Center.

Extremely interesting data, so to say a last "greeting" from ERS-2 …The science community and other users will miss the data which came from this satellite. Right up to the end, ERS data were being recorded and processed in near-real-time at DLR in Neustrelitz, with a focus on maritime security applications. But the Antarctic station will not be idle after the loss of its “midwives” ERS-1 and ERS-2. The German TanDEM-X radar mission with its enormous data volume requires reception capacity in the Antarctic from GARS O'Higgins for eight to nine satellite passages, day after day and year-round.